Field of the Invention
The present invention relates to an imaging lens which forms an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device, and more particularly to an imaging lens composed of seven optical elements which is built in an image pickup device mounted in an increasingly compact and low-profile smartphone, mobile phone or PDA (Personal Digital Assistant), or a game console, or an information terminal such as a PC, or a home appliance with a camera function.
In the present invention, whether or not an optical element is a lens is decided depending on whether or not it has refractive power near an optical axis. An optical element having refractive power near the optical axis is called a lens. An optical element which does not have a lens function can contribute to improvement of aberrations in the peripheral area of an image without changing the overall focal length. In terms of lens surface shape, a convex surface or a concave surface means that the paraxial portion of the surface (portion near the optical axis) is convex or concave. A “pole-change point” on an aspheric surface means a point on the aspheric surface at which a tangential plane intersects the optical axis perpendicularly.
Description of the Related Art
In recent years, there has been a general tendency that many information terminals have a camera function. Also, home appliances with a camera have been introduced into the market. For example, a user away from home can see in real time what is going on at home through the camera mounted in a home appliance by telecommunication between the home appliance and his/her smartphone and can control the home appliance on the smartphone. This kind of home appliance is called a smart home appliance and various smart home appliances such as smart cleaners, smart air conditioners, and smart refrigerators have become widely used. On the other hand, glasses or wrist watches with a camera function as products called wearable terminals have appeared in the market. A variety of unprecedented high value-added products are being developed by adding a camera function to existing products and it seems that products which further enhance consumer convenience or consumer satisfaction will be increasingly developed in the future. The cameras mounted in such products are required not only to provide high resolution to cope with an increase in the number of pixels but also to be compact and low-profile and offer high brightness and a wide field of view. For example, for use in mobile terminals, the imaging lenses are strongly expected to be low-profile and offer high resolution and high brightness. On the other hand, for use in various home appliances, the imaging lenses are strongly expected not only to meet these requirements but also to provide a capability to capture an image of an object over a wide field of view.
However, in order to provide a low-profile imaging lens with high brightness and a wide field of view as described above, the problem of difficulty in correction of aberrations in the peripheral area of an image has to be addressed and unless the problem is addressed, it is difficult to deliver high imaging performance throughout the image.
Conventionally, for example, the imaging lens described in JP-A-2012-155223 (Patent Document 1) is known as an imaging lens designed to achieve compactness and high-resolution.
Patent Document 1 discloses an imaging lens which includes, in order from an object side, a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with positive refractive power, a fourth lens group with negative refractive power, a fifth lens group with positive refractive power, and a sixth lens group with negative refractive power. In this imaging lens, the second lens group is a negative meniscus lens having a convex surface on the object side, an aperture stop which sets a prescribed aperture is located on the object-side surface of the third lens group, the fourth lens group is a negative meniscus lens having a convex surface on an image side, and the sixth lens group has negative refractive power near the optical axis, in which it gradually changes to positive refractive power and the positive refractive power is stronger in a portion nearer to the periphery. Consequently the imaging lens offers a relatively wide field of view, high brightness, and high resolution.
The imaging lens described in Patent Document 1 includes six single lenses or six lenses including one cemented lens, with an F-value of 2.32 to 2.43 for the configuration with six single lenses or with an F-value of 2.09 to 2.35 for the configuration with six lenses including one cemented lens, so that it corrects various aberrations properly and relatively high brightness. In addition, it provides a relatively wide field of view of 66 to 70 degrees. However, the total track length is longer than the diagonal length of the effective imaging plane of the image sensor, so that it is difficult to use the imaging lens in a mobile terminal which is expected to be low-profile. If this lens system is supposed to offer a wider field of view and higher brightness, it would be difficult to correct aberrations in the peripheral area properly and ensure high image quality throughout the image. In addition, if a cemented lens is used, a sophisticated manufacturing technique is required and thus it is difficult to mass-produce the imaging lens to meet the demand for cost reduction.
As mentioned above, in the conventional art, it is difficult to provide a low-profile low-cost imaging lens which offers high brightness, high resolution and a wide field of view.